Rheostat switch



July 4, 1950 M. J. LARSEN ETAL RHEOSTAT SWITCH Filed Nov. 20, 1948Patented July 4, 1950 UNITED `STATES PATENT OFFICE RHEOSTAT SWITCHMerwin J'. Larsen, Villa Park, and James A. Kochi, Evanston,'Ill.,assignors, by mesne assignments, to Central Commercial Industries, Inc.,Chicago, Ill., a corporation of Delaware Application November 20, 1948,Serial No. 61,160

9 Claims. l

This invention relates to electric switches and has particular referenceto switches for use with keying circuits adapted to be interposed in anelectrical network between an audio signal supply source and the poweramplifier of a signal translatingA device,

The primary object of the invention is the provision of a variableresistive switch, the operation of which, in response to depression of aplaying-key, suppresses what has come to be called key thump caused bythe sudden surge of signal voltage when close circuiting a source oftone signal voltage with the input circuit of the power amplier of thesignal translating device.

A further object of the invention is the pro-- vision of a simple,inexpensive electric switch which is sturdy and dependable in operationand will function to vary the effective resistance in a signaltransmission circuit as a function of the velocity of a control key.

A still further object of the invention is the provision of a noiseless,positively acting switch.

Other objects of the invention will manifestly appear from the followingdescription and accompanying drawings, in which:

Figure l is a View partly in side elevation and partly in section of theswitch and coactive control key;

Figure 2 is a view in rear elevation thereof;

Figure 3 is a section on line 3-3 fof Figure 1;

Figure 4 is a View in elevation of the resistive strip beforeapplication of the metallic coating to the respective ends thereof;

Figure 5 is a View similar to Figure 4 of the strip showing the metalliccoating applied thereto;

Figure 6 is a section on lines '6 6 o'f Figure 4, and y Figure 7 is aperspective view of themetallic clip.

Figure 8 is a view in :perspective of a portion of the vertical memberof the switch.

In carrying the `invention into practice, use is made of a channelmember 5 of any suitable well known material such as hard wood,aluminum, Bakelite, or the like. Secured at 6 between the opposite sideflanges 5 of member and disposed at right angles to said member, is amember 'I of electrical insulating material, the same formed at itslower end with a foot 8 confined between said side iianges 5v bytheaforementioned means 6.

Placed iiatwisey against the front face of member 'I is la. flat stripI0 of suitable well known electrical vinsulating material: such` asBakelite.

2 The front face of said strip has parallel, laterally spaced apartlanes or tracks I I, i2 and I3,

each consisting of a resistive coating compound such as carbon, or otherwell known resistive coating, such that each lane or track is (in thepresent example) of the same predetermined high resistance betweenpoints A and B. The lower terminal of each of said lanes merges into alow resistive coating of carbon or other well known resistive coatingindicated at I5, forming the signal voltage output end of the strip fortransmission of signal voltage to the input circuit of a power amplifierfrom a source oi signal voltage common only to the high resistive lanewith which the particular low resistive coating I 5 is electricallycoactive. The opposite ends of said lanes merge into `a carbon coatingI5 which is common to all of said lanes.

Carbon, or other well known resistive coating may be painted or sprayedupon the strip Ill by any well known method and when dried either by airor heat treatment, the low resistive coat- Ving I4 at the upper ends oflanes Il, I2 and I3 and common to all thereof, will preferably haveapplied thereto a metallic conductive paint I6 providing a commongrounding connection to be hereinafter more fully described. The lowresistive coating I5 at the lower end of each separate lane II, I2 andI3 is similarly coated with conductive paint I6. These conductivecoatings may consist of any well known metallic paint which, whenapplied and properly set, insure hard surfaces at the respective ends ofsaid lanes, which said surfaces merge into said lanes so as to formtherewith one perfectly smooth continuous surface which lies in a singleplane.

At the upper end of the vertical portion 'I of rthe switch are terminallugs Il, there being one thereof for each separate high resistive laneIl, I2 or I3. As these lugs are identical both in form and purpose, adescription of one will suilice for all. In Figure l, each separate lugIl' will connect with the output lead I9 of a tone signal Supply source29 such as a vacuum tube oscil- -lator or any well known generator or" awave form of signal voltage of predetermined irequency. Only one lug I'Iis shown connected to a-signal supply source. Assuming that the switchis used with a musical instrument employing octave couplers, one of saidlugs will connect vwith a tone frequency generator producing signalvoltage, the frequency of which corresponds with Aan eight foot pipetone whereas the others of said lugs respectively connect with tonesignal sources providing tone signals corresponding to 4 and 16 ft.tones. Soldered at 2I to said lug is one terminal of a coil spring 2l aproviding a resilient conductive lead between said source of Signalvoltage and the short arm 22 of a contactor 23, to which short arm theother terminal of said coil spring is soldered as at 24.

The rearwardly projecting portion 'la of vertical guide slots 25, therebeing a separate slot for each individual contactor 23. These slots arehorizontally aligned with their coactive resistive lanes I I, l2 and I3on strip IU. The contactors are each thereof mounted for pivotal motionabout a center 26 on an insulating block 21 secured to the back end of alever arm 28, pivoted for vertical motion at 2S to the forward end ofmember 5.

Each contactor has a long arm Sil received in a coactive guide slot inportion 'la of inember 'I and, as shown, the free end of said arm has aneffective upset portion 30d presenting a smooth curved surface foreffective electrical coaction with the resistive coating cf a respectiveone of the aforementioned lanes.

A metallic clip 3l is secured at 32 to member 'I and is formed withparallel springy portions 33 which are rmly pressed against theconductive coating I6 of the respective lanes il, I2 and I3 as shown atFigure 1. Said clip is connected to ground 34.

The springs 21a function to maintain a yielding, yet free slidingconnection between the portion 30a of a respective contacter and itscoactive resistive material on strip- Ill, and to urge lever arm 28 tothe position shown at Figure 1. The forward end of the lever arm isprovided with an adjusting screw 34' which is coactive with the back endof a vertically tiltable playingkey 35 such as the elongated key of apiano, organ or other musical instrument in which the key can pivotabout a horizontal axis. When- `the playing-key is in the position shownat rFigure 1, screw 34 rests upon a strip of felt 34a,

at the bac-k end of the key and when pressure is applied to the forwardend of the key, motion is ktransferred to the contactors 23simultaneously moving them ofi' of the low resistive common Vground I8and progressively onto their high resistive coatings II, I2 and I3, thevalue of the resistance lowering progressively as the oontactorsapproach their low resistive output terminals I6. Each low resistiveoutput coating has firmly connected thereto a terminal lead 35 adaptedto be connected to the input circuit of the power amplier of a soundtranslating device.

Emphasis is placed upon the'structural features of the switch and theforms and shapes of the component elements to the end that smoothness inaction is assured and whereby variations in the resistance is madewithout any break in' 'the continuity of the movable Contact element ofthe switch during motion of said contactor rectilinearly. By mountingthe contactors 23 as shown and described herein, only a negligibledegrees of pressure of the effective contact porytions 30a is required'for a good electrical contact between same and the resistive coating ofthe insulating strip I0. By proper hardening of the coating materials I6and I5 and the resistive lcoatings which smoothly merge thereinto and byestablishing and maintaining only a very minimum amount of pressurebetween same and said effective contact portions 30a, to which may beadded use of German silver or its equivalent `and of suitable gauge, theresistive surfaces are kept clean and the switch is perfectly noiselessover very long periods of time. In practice. we nd that by theseniceties, rubbing engagement of the portions 30al of contactors 30 canbe continued for a great number of operations without fear of any metalbeing transferred onto the resistive lanes from said portions 30a.

In order to cushion the action of the lever 28, said lever is preferablyprovided with a strip of felt 28a adapted to come against the under sideof the foot of the member 1 when the lever 28 is returned to an olfposition of the switch. While it is preferred to coat the low resistivecommon ground end I4 of strip l0 with a metallic paint and similarly toapply metallic coatings I6 to the opposite ends of the lanes Il, I2 andI3, `we do not wish, however, to be limited in this respect as testshave been shown that very satisfactory results are had simply by the lowresistance ycarbon coatings when used alone. By heat treatment of stripI0 after application of the carbon thereto, the resistive surface thusformed becomes exceedingly hard and is relatively permanent. Thecontactors 23 give excellent results when made from German silver ofsuitable gauge. With the resistive surface characterized asaforementioned a positively acting switch is produced which is smooth inaction, noiseless and sturdy. The high resistive zone in each of theindividual rectilinear lanes may be of different resistance fordifferent purposes but lwhen used as a high impedance switch, said zonewill preferably have' a resistance of approximately 50,000 ohms, suchthat when the contactor leaves the grounded terminus I and enters andproceeds in a direction longitudinally of the intermediate highresistive zone and toward terminus I6', the change in resistance will atno time be by `jumps but to the contrary will progressively lower as thecontacter approaches said terminus IS.The reversal of this operationoccurs during return motion of the contactor in the direction of andback onto the common grounding terminus.

The standard width of the playing-key of an organ or a pianoisapproximately one-half inch. As the herein disclosed switch organizationis designed and constructed primarily for use with such keyboard musicalinstruments, an essential feature of our invention is the embodiment ina unitary structure of a'plurality of individual switches which are soformed and arranged as to enable the entire organization to beaccommodated in a space not in excess of one-half inch. On `reference toFigure 2`of the accompanying drawings, it is noted that the organizationoccupies a' vertical position above playingkey 35, and that the width ofthe playing-key is slightly inexcess of the width of said organization.It is further noted that motion ofthe key is simultaneously transferred'to all contact elements While they are in'constant yielding engagementwith their coactive resistivelanes, the distance of travel of allcontactors being precisely the same.' This is necessary in order thatthe impedance in the separate output paths through said lanes shall bethe same ineach thereof. yTo the best olf our knowledgeno one has everbefore combined in a single 4resistive switch organizationaplurality vofindividual switches lcapableiof being accommodated in so limited a spaceas allowed for their association with the playing-key of an'r organ orsimilar keyboard musical instrument! This is made possible by'our use ofa. thin, stiif rectangular strip of insulating material, one side ofwhich is coated with a hard resistive substance as herein set forth.This enables use of fine gauge contact wires such as 30, which wires arein parallel relation and only slightly spaced apart from each otherlaterally. Also by our use of a single strip of insulating material, allresistive lanes necessary for as many conductive paths can be readilyinsulated from each other by simply routing the material to a depthbelow the effective resistive coating substance after one face of thestrip has been coated with the resistive substance.

What we claim as our invention is:

1. An electric switch organization for actuation by a yplaying key of amusical instrument comprising means having a plurality of parallelresistive lanes disposed in a common plane and each including a highresistive zone, a low resistive zone and grounding means common to saidhigh resistive zones, a plurality of contactors coacting with therespective lanes and movable progressively from said common ground andlongitudinally along said high resistive zones, and into and along saidlow resistive Zones, voltage output terminals for the respective lowresistive zones, and means including voltage input terminals andexpansible and contractable rnetallic conductors connecting saidterminals with the respective contactors.

2. In an electric switch for varying the resistance in a signaltransmissionpath and actuable by an elongated playing-key which haspivotal motion about a horizontal axis; insulating material having alongitudinal resistive surface; means by which said material is adaptedto be mounted to dispose said resistive surface at right angles to thelongitudinal center line of said key; and means including a playing-keyactuated icontactor coactive with said resistive surface to vary theresistance in said path as a function of pivotal motion of said key.

3. In an electric switch for varying the resistance in a signaltransmission path and actuable by an elongated playing-key which haspivotal motion about a horizontal axis; a thin planar strip ofinsulating material lcoated at one side with a resistive materialproviding a longitudinal resistive surface; means by which said strip isadapted to be :mounted to dispose said resistive surface at right anglesto the longitudinal center line of said key; and means electricallycoactive with said resistive surface in response to motion of said keyfor varying the resistance in said path.

4. In a variable resistive switch for use in a signal transmission pathemploying a control key which is tiltable about a horizontal axis; astrip of insulating material provided with a longitudinal resistivesurface; a springy contactor in constant engagement "with said resistivesurface and movable longitudinally thereof; and mounting means for saidstrip for disposing said resistive surface at right angles to the longaxis of said key and including means coactive with said key for causingrelative motion between said surface and said contactor with resultantprogressive changes in the effective resistance in said path duringpivotal motion of'said key.

5. As a new article of manufacture, a variable resistive switchorganization comprising a flat rectangular strip of electricalinsulating material having a plurality of laterally spaced apart,rectilinearly disposed, parallel continuous, resistive lanes at one sidethereof, said lanes being electrically insulated from each other andadapted to be connected to respective signal transmission paths; meansincluding a slidably mounted contacter for and in constant yieldingengagement with each of said lanes; and an actuator common to saidcontactors for simultaneously sliding same along said lanes to vary theimpedance in said paths.

6. As a new article of manufacture, a variable resistive switchorganization comprising a flat rectangular strip of electricalinsulating material having a plurality of laterally spaced apartrectilinear, continuous lanes at one side thereof, said lanes beingelectrically insulated from each other and adapted to be connected torespective signal transmission paths; means including a resilientcontactor :for and in constant yielding electrical engagement with eachof said lanes; and an actuator common to all of said contactors forcausing relative rectilinear motion between said strip and saidcontactors for simultaneously varying the impedance in said paths byeven increments.

7. A switch organization as set forth in claim 5 wherein the impedancevalue of the resistance in each of said lanes is the same.

8. A switch organization as set forth in claim 6 wherein the impedancevalue of the resistance of each of said lanes is the same.

9. The combination with a. playing key of a musical instrument, of aswitch organization including means operated by said key and disposedperpendicular to a line drawn through the long axis thereof, saidorganization having a width not in excess of that of said key andincluding a vertically disposed, thin rectangular strip of insulatingmaterial secured flatwise against said means and adapted to be connectedto a signal transmission path, and a springy contactor slidablerectilinearly on said strip and actuable by said playing-key to causesliding motion of said contacter between the ends of said strip, saidstrip having a continuous resistive coating substance against thesurface of which the contactor has constant yielding engagement.

MERWIN J. LARSEN. JAMES A. KOEHL.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 432,894 Parrish July 22, 18902,135,809 Fruth Nov. 8, 1938 2,215,124 Kock et al. i Sept. 17, 19402,242,327 Rubinstein May 20, 1941 2,280,305 Schauer Apr. 21, 1942

